This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. 1. Many proteins are transported after their synthesis across the bacterial plasma membrane. They are moved by the SecA ATPase through a translocation channel that is formed by a heterotrimeric membrane protein complex, called the SecYEG complex. Structures of the SecA ATPase had been determined before and a structure of the channel from an archaebacterium had also been determined. The next step was therefore to determine the structure of the SecA ATPase bound to the channel. 2. SecA is the ATPase that is responsible for the post-translational translocation of polypeptides across the SecY channel in bacteria. Several SecA structures have been determined, but a full understanding of the function of the ATPase has not yet been achieved. One major question concerns the role of a SecA dimer. We had obtained data that suggested that SecA functions as a monomer, while others have claimed that it functions as a dimer. In addition to resolving this issue, we became interested in the structure of Thermotoga maritima SecA because we had crystallized the SecA from this species in complex with the channel (see above). The question was whether T. maritima SecA undergoes conformational changes upon binding to the channel.